Support Elliptic curve signatures in JWT module

[ptone]

Currently

https://github.com/GoogleCloudPlatform/google-auth-library-python/blob/master/google/auth/crypt.py

Only supports RSA

It would be great to add support for ES* such as ES256 signing algorithms as well

[dhermes]

@ptone Can you point to some docs for Google Auth services that accept elliptic curve signed JWTs?

[theacodes]

@dhermes confirmed internally that a new (unannounced) service will support these. This isn't critical priority, but we should do this.

[engelke]

Cloud IAP provides JWT signed with ES256 only.